Scientific usage: A blood agar plate used to culture bacteria and diagnose infection.

Agar (pronounced /ˈɑːɡər/, "AH-gər") or agar-agar (/ˈɑːɡərˈɑːɡər/, "AH-gər-AH-gər") is a jelly-like substance, obtained from algae[1] and discovered in the late 1650s or early 1660s by Mino Tarōzaemon (美濃 太郎左衛門) in Japan, where it is called kanten.

Agar is derived from the polysaccharideagarose, which forms the supporting structure in the cell walls of certain species of algae, and which is released on boiling. These algae are known as agarophytes and belong to the Rhodophyta (red algae) phylum.[2][3] Agar is actually the resulting mixture of two components: the linear polysaccharide agarose, and a heterogeneous mixture of smaller molecules called agaropectin.[4]

The gelling agent in agar is an unbranched polysaccharide obtained from the cell walls of some species of red algae, primarily from the generaGelidium and Gracilaria. For commercial purposes, it is derived primarily from Gelidium amansii. In chemical terms, agar is a polymer made up of subunits of the sugar galactose.

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Agar was discovered around 1658 in Japan (where it is called kanten, coined from the phrase kan-zarashi tokoroten or “cold-exposed agar”) by Mino Tarōzaemon, an innkeeper who was said to have discarded some surplus seaweed soup and noticed that it gelled later after a winter night's freezing.[6]

Agar was first used in microbiology in 1882[7] by the German microbiologist Walther Hesse, an assistant working in Robert Koch's laboratory, on the suggestion of his wife Angelina Fannie Eilshemius Hesse. He discovered that it was more useful as a solidifying agent than gelatin, due to its better solidifying temperature.[8][9][10]

Agar was first subjected to chemical analysis in 1859 by the French chemist Anselme Payen, who had obtained agar from the marine algae Gelidium corneum.[11]

Agar consists of a mixture of agarose and agaropectin. Agarose, the predominant component of agar, is a linear polymer, made up of the repeating monomeric unit of agarobiose. Agarobiose is a disaccharide made up of D-galactose and 3,6-anhydro-L-galactopyranose. Agaropectin is a heterogeneous mixture of smaller molecules that occur in lesser amounts, and is made up of alternating units of D-galactose and L-galactose heavily modified with acidic side-groups, such as sulfate and pyruvate.[12][13][14]

Agar exhibits hysteresis, melting at 85 °C (358 K, 185 °F) and solidifying from 32–40 °C (305–313 K, 90–104 °F).[15] This property lends a suitable balance between easy melting and good gel stability at relatively high temperatures. Since many scientific applications require incubation at temperatures close to human body temperature (37 °C), agar is more appropriate than other solidifying agents that melt at this temperature, such as gelatin.

The word "agar" comes from agar-agar, the Malay/Indonesian name for red algae (Gigartina, Gracilaria) from which the jelly is produced.[16] It is also known as Kanten, Japanese isinglass, Ceylon moss or Jaffna moss.[17]Gracilaria lichenoides is specifically referred to as agal-agal or Ceylon agar.[18]

An agar plate or Petri dish is used to provide a growth medium using a mix of agar and other nutrients in which microorganisms, including bacteria and fungi, can be cultured and observed under the microscope. Agar is indigestible for many organisms so that microbial growth does not affect the gel used and it remains stable. Agar is typically sold commercially as a powder that can be mixed with water and prepared similarly to gelatin before use as a growth medium. Other ingredients are added to the agar to meet the nutritional needs of the microbes. Many specific formulations are available, because some microbes prefer certain environmental conditions over others. Agar is often dispensed using a sterile media dispenser.

As a gel, an agar or agarose medium is porous and therefore can be used to measure microorganism motility and mobility. The gel's porosity is directly related to the concentration of agarose in the medium, so various levels of effective viscosity (from the cell's "point of view") can be selected, depending on the experimental objectives.

A common identification assay involves culturing a sample of the organism deep within a block of nutrient agar. Cells will attempt to grow within the gel structure. Motile species will be able to migrate, albeit slowly, throughout the gel and infiltration rates can then be visualized, whereas non-motile species will show growth only along the now-empty path introduced by the invasive initial sample deposition.

Another setup commonly used for measuring chemotaxis and chemokinesis utilizes the under-agarose cell migration assay, whereby a layer of agarose gel is placed between a cell population and a chemoattractant. As a concentration gradient develops from the diffusion of the chemoattractant into the gel, various cell populations requiring different stimulation levels to migrate can then be visualized over time using microphotography as they tunnel upward through the gel against gravity along the gradient.

Research grade agar is used extensively in plant biology as it is supplemented with a nutrient and vitamin mixture that allows for seedling germination in Petri dishes under sterile conditions (given that the seeds are sterilized as well). Nutrient and vitamin supplementation for Arabidopsis thaliana is standard across most experimental conditions. Murashige & Skoog (MS) nutrient mix and Gamborg's B5 vitamin mix in general are used. A 1.0% agar/0.44% MS+vitamin dH2O solution is suitable for growth media between normal growth temps.

The solidification of the agar within any growth media (GM) is pH-dependent, with an optimal range between 5.4-5.7. Usually, the application of KOH is needed to increase the pH to this range. A general guideline is about 600 µl 0.1M KOH per 250 ml GM. This entire mixture can be sterilized using the liquid cycle of an autoclave.

This medium nicely lends itself to the application of specific concentrations of phytohormones etc. to induce specific growth patterns in that one can easily prepare a solution containing the desired amount of hormone, add it to the known volume of GM, and autoclave to both sterilize and evaporate off any solvent that may have been used to dissolve the often-polar hormones. This hormone/GM solution can be spread across the surface of Petri dishes sown with germinated and/or etiolated seedlings.

Agar-agar is a natural vegetable gelatin counterpart. White and semi-translucent, it is sold in packages as washed and dried strips or in powdered form. It can be used to make jellies, puddings, and custards. For making jelly, it is boiled in water until the solids dissolve. Sweetener, flavouring, colouring, fruit or vegetables are then added and the liquid is poured into molds to be served as desserts and vegetable aspics, or incorporated with other desserts, such as a jelly layer in a cake.

Agar-agar is approximately 80% fiber, so it can serve as an intestinal regulator. Its bulk quality is behind one of the latest fad diets in Asia, the kanten (the Japanese word for agar-agar[3]) diet. Once ingested, kanten triples in size and absorbs water. This results in the consumers feeling more full. This diet has recently received some press coverage in the United States as well. The diet has shown promise in obesity studies.[20]

One use of agar in Japanese cuisine (Wagashi) is anmitsu, a dessert made of small cubes of agar jelly and served in a bowl with various fruits or other ingredients. It is also the main ingredient in mizu yōkan, another popular Japanese food.

In Philippine cuisine, it is used to make the jelly bars in the various gulaman refreshments or desserts such as sago gulaman, buko pandan, agar flan, halo-halo, and the black and red gulaman used in various fruit salads.

In Vietnamese cuisine, jellies made of flavored layers of agar agar, called thạch, are a popular dessert, and are often made in ornate molds for special occasions. In Indian cuisine, agar agar is known as "China grass" and is used for making desserts. In Burmese cuisine, a sweet jelly known as kyauk kyaw(ေကျာက်ေကြာ) [tɕaʊʔtɕɔ́]) is made from agar.

In Russia, it is used in addition or as a replacement to pectin in jams and marmalades, as a substitute to gelatin for its superior gelling properties, and as a strengthening ingredient in souffles and custards. Another use of agar-agar is in ptich'ye moloko (bird's milk), a rich jellified custard (or soft meringue) used as a cake filling or chocolate-glazed as individual sweets. Agar-agar may also be used as the gelling agent in gel clarification, a culinary technique used to clarify stocks, sauces, and other liquids.